After a 38-year period of calm—the longest in its recorded history—Hawaii’s Mauna Loa has reawakened.
At approximately 11:30 p.m. local time on Sunday, Mauna Loa, the largest active volcano in the world, began erupting. Lava oozed into Moku‘āweoweo, the bowl-like summit of the volcano, staining the blue-black sky with crimson hues. Throughout the night, the lava emerged and flowed mostly within this caldera, with a small amount spilling over the side. But as the sun rose, molten rock was spotted bleeding out of fresh cracks on the volcano’s northeastern flanks—a section of the mountain that is slowly being pulled apart.
At present, no major population centers are threatened and no evacuation orders have been issued, but the situation is rapidly evolving, and eruptions at Mauna Loa have proven unpredictable in the past.
“There are eruptions at Mauna Loa that end in a day. There are also eruptions that go on for a long time. Really, there is just no way to know right now,” says Jackie Caplan-Auerbach, a volcano seismologist at Western Washington University.
For the time being, Mauna Loa seems to be sticking to a familiar pattern. “It started at the summit, and it really quickly moved to the rift zone,” says Wendy Stovall, a volcanologist with the U.S. Geological Survey’s Volcano Hazards Program. “The next really typical thing for it to do is stay in that rift zone and not move anywhere else on the volcano.”
This is a day that volcanologists, particularly those at the USGS Hawaiian Volcano Observatory, have spent decades anxiously preparing for. Now that the eruption has begun, a particularly destructive scenario does not yet appear to be unfolding, allowing researchers to breathe a tentative sigh of relief.
“Nobody is happy this happened, but it’s almost like watching a horror movie after the jump scare has happened,” says Brett Carr, a volcanologist at the University of Arizona. “Now at least we know what’s going on.”
Fires of the Long Mountain
Like its Hawaiian cousins, Mauna Loa owes its existence to a superheated plume of matter rising from the depths of Earth’s mantle to the base of the Pacific tectonic plate. As that plume reaches the plate, it decompresses and the material begins to melt, creating vast reservoirs of magma. And as the Pacific plate has shifted over time, a chain of volcanoes fueled by this subterranean blowtorch has risen in its wake.
Ascending 10.5 miles from its seafloor base and covering 2,000 square miles, Mauna Loa, which means “Long Mountain” in Hawaiian, is particularly colossal. It is also hyperactive. Eruptions have been observed for centuries ever since the Hawaiian archipelago was settled by seafaring Polynesians. Since 1843, when detailed written records began, there have been 33 documented eruptions.
The magma’s chemistry has been mostly consistent since that time, resulting in runny, extremely hot lava flows (up to 1,800 degrees Fahrenheit) and a dearth of any especially explosive activity. In the distant past, eruptive activity was concentrated either at Mauna Loa’s summit or on its slopes. But the most recent 33 all began at the summit, and half of those stayed safely confined to the Moku‘āweoweo caldera.
In some eruptions, however, lava has unpredictably emerged from one of two rift zones on Mauna Loa’s flanks. If lava is released from one of these areas, known as the Southwest Rift Zone, it can represent an extremely dangerous scenario, with steep slopes capable of channeling lava directly into nearby residential areas, such as the districts of Ka‘ū and South Kona, in a matter of hours.
When lava emerges from the Northeast Rift Zone, however, as is the case in this eruption, the situation is a little less precarious. The city of Hilo is below, and it has been repeatedly threatened by lava incursions in the past, but the slopes above it are gentle, and the most active part of the rift zone sits miles away. If lava continues to flow from the Northeast Rift Zone for several weeks, there is a chance that molten rock could reach Hilo, but scientists would know if this was going to happen well in advance, giving residents ample time to evacuate.
“Fortunately, we aren’t looking at a Southwest Rift Zone scenario,” Carr says. “That’s a scenario where loss of life could occur.”
The beast awakens
Even though it is one of the most closely monitored volcanoes on Earth, Mauna Loa has yet to give up most of its secrets. Despite some common patterns, each eruption has been novel in some way.
A 1859 eruption lasted for 300 days and produced a jaw-dropping 32-mile-long flow, destroying villages and vital resources. An eruption 91 years later lasted for only 23 days, but it dumped 491 million cubic yards of lava onto Hawaii’s Big Island and destroyed some of the island’s infrastructure. The most recent eruption before now, in 1984, came close to engulfing Hilo—a scenario that, in the past, officials have attempted unsuccessfully to prevent by using explosives to divert Mauna Loa’s lava flows.
Since 2019, Mauna Loa has been twitching considerably. It has been changing shape and quaking sufficiently to suggest that magma was churning within. This past September, the unrest became even more heightened, leading researchers to suspect that magma was being pumped into a reservoir at the summit. Hawaiian civil defense members held meetings shortly after to prepare residents for a possible emergency scenario.
Then, shortly before magma breached the surface on November 27, the volcano began to quake furiously. “There was about an hour of really elevated seismicity as magma moved from the summit storage region out to eruption,” Stovall says. “So that was all the warning that we really had.”
Why did it take nearly four decades to erupt again—and why now? Researchers will have to continue to study the eruption and its aftermath to try to find out. Clearly, something changed within the volcano, but the trigger is currently unknown. “Things just hit a point where something fails,” says Diana Roman, a geophysicist at the Carnegie Institution for Science in Washington, D.C.
The absence of warning signs in the days prior to the eruption was not unusual for Mauna Loa. Nor was the beginning of the fireworks, which occurred harmlessly at the summit. But then staff at the Hawaiian Volcano Observatory flying over the volcano spied lava emerging from its northern flanks at 6:30 a.m. local time on November 28. Now the only thing to do is watch and see how the situation unfolds.
So far, so good
As Mauna Loa’s past paroxysms have demonstrated, it is hard to tell how a given eruption may evolve. Explosive activity at the summit, which can produce ephemeral ash plumes, is possible although unlikely during this eruption. The primary threats are the lava flows from the volcano’s flank, and whether they become destructive depends on how abundant and persistent they turn out to be.
Lava may cut across Saddle Road, which runs across the island, and it may endanger parts of the nearby Mauna Loa Observatory. If so, that would be unwelcome, but it is a far cry from the most devastating possibilities.
The ongoing threat to life and property should not be discounted, Stovall says, but “there is relief” among volcanologists, “especially because the worst-case scenario is not the scenario we have.”
For now, the U.S. Geological Survey and its partners will continue to monitor the volcano around the clock—and as soon as possible, scientists will be deployed on-site, setting up equipment while grabbing samples of the lava to see how its composition—and its potential for destructive blasts—changes as the eruption unfolds.
So far, Stovall says, Mauna Loa “is behaving well.” The hope is that it continues to do so. “We’re only at the beginning.”
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